Crazy as a loon

: : What in (English-speaking) Europe is called the Great Northern Diver is called in North America the Common Loon (a species of bird). It has a weird, haunting cry which presumably suggested to someone the howls of the insane. Hence, crazy as a loon.

: : DFG

: One reason that that odd, sort of laughing, sound made by loons makes such an impression is that it sometimes does it at night, when most birds are silent. Kinda spooky. SS

not forgetting to mention that 'loon' was a term of abuse in Shakesperean times, being as it is a shortened form of 'lunatic' i.e. somebody made mad by the power of the moon.